On writing poetry
From W. H. Auden in The Dyer's Hand:
"Rhymes, meters, stanza forms, etc., are like servants. If the master is fair enough to win their affection and firm enough to command their respect, the result is an orderly happy household. If he is too tyrannical, they give notice; if he lacks authority, they become slovenly, impertinent, drunk and dishonest."
On writing fiction
B.C. Southam, in his study of Jane Austen's Literary Manuscripts, spoke about what fiction is (or should be), and quoted from the Preface to Joseph Conrad's book, The Nigger of the Narcissus:
"As Conrad saw it, the art of fiction is to seize 'a passing phase of life . . . to show its vibration, its colour, its form; and through its movements, its form, and its colour, reveal the substance of its truth — disclose its inspiring secret: the stress and passion within the core of each convincing momemnt.'"
On writing, in general
Only as it's National Poetry Month, this last bit of advice comes in the form of a poem from Miss Emily Dickinson:
Tell all the Truth but tell it slant—
Success in Circuit lies
Too bright for our infirm Delight
The Truth's superb surprise
As Lightning to the Children eased
With explanation kind
The Truth must dazzle gradually
Or every man be blind—